First Accurate Experimental Study of Mu Reactivity from a State-Selected Reactant in the Gas Phase: the Mu + H2{1} Reaction Rate at 300 K

This paper reports on the experimental background and methodology leading to recent results on the first accurate measurement of the reaction rate of the muonium (Mu) atom from a state-selected reactant in the gas phase: the Mu + H-2{1} -> MuH + H reaction at 300 K, and its comparison with rigorous quantum rate theory, Bakule et al (2012 J. Phys. Chem. Lett. 3 2755). Stimulated Raman pumping, induced by 532 nm light from the 2nd harmonic of a Nd: YAG laser, was used to produce H-2 in its first vibrational (v = 1) state, H-2{1}, in a single Raman/reaction cell. A pulsed muon beam (from 'ISIS', at 50 Hz) matched the 25 Hz repetition rate of the laser, allowing data taking in equal 'Laser-On/Laser-Off' modes of operation. The signal to noise was improved by over an order of magnitude in comparison with an earlier proof-of-principle experiment. The success of the present experiment also relied on optimizing the overlap of the laser profile with the extended stopping distribution of the muon beam at 50 bar H-2 pressure, in which Monte Carlo simulations played a central role. The rate constant, found from the analysis of three separate measurements, which includes a correction for the loss of H-2{1} concentration due to collisional relaxation with unpumped H-2 during the time of each measurement, is k(Mu) {1}= 9.9[(-1.4)(+1.7)] x 10(-13) cm(3) s(-1) at 300 K. This is in good to excellent agreement with rigorous quantum rate calculations on the complete configuration interaction/Born-Huang surface, as reported earlier by Bakule et al, and which are also briefly commented on herein.